Electrical coupling device



Dec. 20, 1938. B. R. SCHOFIELD 2,140,770

ELECTRICAL COUPLING DEVICE Filed May 19, 1957 3 Sheets-Sheet l INVENTOR BMA/Afm R. scf/@na 'BY/WWK@ ATTO R N EY Dec. 2o, 193s.

B. R. SCHOFIELD ELECTRICAL COUPLING DEVICE Filed May 19, 1937 3 Sheets-Sheet 2 INVENTOR 55E/VAE@ E. ScHoF/ELD.

ATTORNEY Dec. 20, 1938. B, R. scHoFlELD 2,140,770

ELECTRICAL COUPLING DEVICE Filed May 19. 1937 3 Sheets-Sheet 3 52 L-J IN'vENToR 55m/AEDE. cHoF/ELD.

ATTORNEY Patented Dec. 20, 1938 PATENT OFFICE ELECTRICAL CoUPLIN DEVICE Bernard R. Schofield, Chicago, Ill., assignor to Johnson Laboratories, Inc., Chicago, Ill., a corporation of Illinois Application May 19, 1937, serial No. 143,571

Claims.

This invention relates to improvements in highfrequency systems, and more specifically to improvements in high-frequency resonant systems. Resonant systems of thel type herein contemplated are generally employed to link the output circuit of a first vacuum tube to the input circuit'of a second vacuum tube, are usually intended for operation over a limited range of frequencies, and are commonly employed in the intermediate-frequency amplifier of a radio receiver of the superheterodyne type.

An object of this invention is to provide an improved type of resonant system having a bandpass selectivity characteristic with broad response to the desired signal accompanied by increased discrimination against undesired signals on nearby and adjacent channels, but nevertheless being eas'ily and readily adjustable to resonance with a desired signal.

Another object of this invention is to provide a resonant system which may be aligned at a single frequency but which in operation provides substantially uniform performance over a narrow range of frequencies.

Another object of this invention is to provide a resonant system which may be aligned at a single frequency and which in operation provides substantially uniform performance over a narrow range of frequencies extending equally above and below the frequency at which the system is aligned.

Another object of this invention is to provide a resonant system which may be aligned at a single frequency and which in operation provides substantially uniform performance over a narrow range of frequencies, the limits of which may be made variable with respect to the frequency at which the system is aligned, by the action of an external circuit appropriately connected.

The over-all selectivity characteristic of a superheterodyne radio receiver depends to a very considerable extent upon the selectivity characteristic of the intermediate-frequency amplifier. This depends, in turn, upon the arrangement of the resonant circuits which are employed between the vacuum tubes. In coupling devices of the prior art, consisting of two tuned circuits which have a fixed or adjustable degree of 1nductive coupling between them, if thedegree of coupling is at a value designated as the critical value, and the two tuned circuits have been adjusted to resonate at the intermediate frequency, the resultant selectivity characteristic will have a pronounced single peak and gradually sloping sides. This peak indicates that the receiver Will attenuate the higher modulation frequenciesl so that the output of the receiver will sound muted. The gradually sloping sides of the selectivity characteristic indicate that the receiver will not discriminate satisfactorily against undesired signals on nearby channels.

If the receiver is to provide high fidelity of reproduction without sacrifice in ability to discriminate against stations onadjacent or nearby channels, the selectivity characteristic must have a broad and substantially flat top and steeply sloping sides. When the coupling of a conventional two-circuit coupling device is increased beyond the critical point, a double-peaked selectivity characteristic is obtained, but the top has a pronounced central depression and the sides of the characteristic are gradually sloped. Thus the characteristic obtainable from an overcoupled two-circuit coupling device is, at best, only a rough approximation of the desired broad nat-topped and steep-sided selectivity characteristic.

In a preferred embodiment of the present invention, in order to make the sides of the selectivity characteristic sufliciently steep, three highly eicient resonant circuits are employed. When all three mutual couplings are adjusted below the optimum point, the resultant selectivity characteristic has steep sides and a very high and narrow peak. Such an'adjustment is not satisfactory for use in a radio receiver intended for high-fidelity reproduction of voice and music. It is necessary that the steep sides of the characteristic be retained, but that the peak be widened suiiiciently to avoid attenuation of the higher modulation frequencies, if the resultant characteristic is to be of the form required for high fidelity of reproduction.

Previous attempts to attain a reasonably satisfactory selectivity characteristic have involved the use of complicated and expensive circuits, with accompanying difficulty in obtaining initial alignment. As against this, the present invention solves the problem of providing a symmetrical, broad flat-topped, steep-sided selectivity characteristic, by an arrangement which is compact and inexpensive, and in which the initial alignment is lreadily obtained in a simple and denite manner. This arrangement may be used to advantage in any inductively coupled device consisting of two or more coils, and has particular advantages when two or more of the devices are to be operated in cascade, as in the intermediate ampliiier of a superheterodyne receiver.

A preferred embodiment of the present invention provides the proper amount of coupling between three tuned circuits to produce a selectivity characteristic which closely approaches the ideal response, and means are provided to insure the attainment of this characteristic with Y only a simple initial adjustment of the system.

In accordance with the invention, switching means are provided for varying the selectivity characteristic of the system. With the switch in one position, the tuned circuits are over-coupled to provide the desired symmetrical, broad fiat-topped, steep-sided characteristic which the system is intended to provide. 'I'hrowing the switch to the other position, however, provides substantially critical coupling between the three tuned circuits of the device, and -thus permits each of the circuits to be initially aligned accurately to the desired resonant frequency. This feature of the invention makes it possible to attain proper alignment of the system without the use of a cathode-ray oscilloscope and variablefrequency oscillator or other means for securing a visual indication of its selectivity characteristic. Substantially perfect alignment may be realized merely by employing a simple high-irequency oscillator or signal generator adjusted to produce a signal having the frequency for .which the system is to be aligned.

'I'he inductors are preferably of the type having self-supporting windings; and ferromagnetic cores, in order to concentrate the magnetic fields and thereby facilitate the design of a. compact and readily shielded device. The low losses of this type of inductor are of material assistance in obtaining the desired selectivity characteristic. It will be understood that other types of inductor may be employed within the scope of the present invention, but not without some sacrifice in the compactness and emciency of the resonant system.

I'he invention will be better understood by reference to the accompanying drawings, in which:

Fig. 1 is an elevation, partly in section, of an illustrative embodiment of the invention;

Fig. 2 is an elevation, partly in section, taken at right angles to Fig. 1;

Fig. 3 is a schematic circuit diagram illustrating the use of the device of Figs. 1 and 2 in a vacuum-tube amplifying circuit; and

Figs. 4-9 inclusive are circuit diagrams of various modiiications of the arrangements of Fig. 3.

Referring to Fig. l, coils I, 3 and 5 are mounted on insulating tube 1, and cores 2, 4 and 6 are positioned within tube 1 and centrally with respect to coils I, 3 and 5 respectively. Coils I, 3 and 5 are wound in the same direction, and form, with cooperating cores 2, 4 and 6 respectively, inductors I-2, 3-4 and 56. Tube 1 is supported by means of brackets 8 which are attached to insulating base 9, which in turn is secured to one side of shield can I0 by nuts II. Base 9 carries adjustable capacitors I2, I3 and I4 and switch I5, which may be actuated from without shield can I0 through holes I6 therein. Shield can I0 may be attached to chassis I1 by any suitable means, as for example spade bolts I8. Control-grid lead I9 extends through a hole at the top of shield can I0, and the other leads to the device pass through a hole in chassis I1. This type of construction provides a device which is compact, readily assembled on the chassis of a. radio receiver ,or other similar apparatus, and fully shielded when so mounted.

Pig. 2 makes clear the arrangement of single- 2,140,770 v f v.

pole double-throw switch I5, which is provided with two contacts 2I and 22. This figure also shows how the leads from the coils I, 3 and 5 are attached to terminals 23 of capacitors I2, I3 and I4, which are arranged to protrude through insulating base 9, to form three resonant circuits 28, 29 and 30 (see Fig. 3). A coupling winding 24 consisting of a small number -of turns is wound `over coil 3 in the same direction, and bucking winding 25, consisting of a fraction of one turn, is wound over and in the opposite direction to coil 5. Windings 24 and 25 are connected in series between contacts 2| and 22 of switch I5, so that when switch I5 is in the position shown, these windings are in series with inductor 5 6. When switch I5 is thrown to its other position, however, tuned circuit does not include windings 24 and 25.

Fig.- 3 shows schematically the use of the device of Figs. l and 2 between vacuum tubes 26 and 21. Inductor I-2 is in the plate or output circuit of vacuum tube 26, and inductor 5--6 is included in the grid or input circuit of vacuum tube 21. It will be understood that either of vacuum tubes 26 and 21 may be arranged to function otherwise than as an amplifier. When switch I5 is on contact 22, windings 24 and 25 are also included in the input circuit of vacuum tube 21. The capacitive coupling between tuned circuits 28 and 29 is minimized by so arranging the leads that the low radio-frequency potential terminal of capacitor I2 is adjacent the low radiofrequency potential terminal of capacitor I3. The capacitive coupling between tuned circuits 29 and 30 likewise is minimized by having the low radio frequency potential terminal of capacitor I4 adjacent the low radio frequency potential terminal of capacitor I3. 'I'he capacitive coupling between tuned circuits 28, 29 and 30 is minimized also by so arranging the connections that the finish leads of coils I, 3 and 5 are at low .radio-frequency potential. The latter precaution greatly reduces the capacitive coupling which would otherwise occur due to the proximity of windings 24 and 25 to coils 3 and 5 respectively.

With switch I5 on contact 2|, inductors I-2 and 5-6 are so spaced upon tube 1 that the coupling between circuit 29 and each of circuits 28 and 30 is substantially critical. With this degree of coupling, the selectivity characteristic of the device has a single peak, and each of the tuned circuits 28, 29 and 30 is readily aligned to the desired resonant frequency by adjustment of capacitors I2, I3 and I4 respectively, each capacitor being varied separately until the maximum response is obtained for a given input voltage.

Switch I5 is now thrown to contact 22, thus connecting windings 24 and 25 in series with inductor 5-6 in tuned circuit 38. 'I'he function of coupling winding 24 is to produce over-coupling between tuned circuits 29 and 30, in such a manner that these two circuits taken alone would produce a double-peaked selectivity characteristic having a desired spacing between the peaks. 'Ihe single-peaked characteristic due to the critical coupling between circuits 28 and 29, however, remains practically unchanged, with the result that the over-all selectivity characteristic of the device is of the desired flat-topped, steepsided form capable of passing a desired band width.

In this manner, the device may be perfectly aligned without diiculty and without the use of any special response-curve indicating apparatus such as isusually required for the proper alignment of coupling devices capable oi' passing a band of substantial Width.

The function of bucking winding 25 is to pre-v vent the resonant frequency for which tuned circuit 30 is adjusted from being changed when coupling Winding 24 is inserted in the circuit. Since bucking winding 251s wound in the opposite direction to coil 5, it produces a decrease in the inductance of the circuit just suiiicient to oppose the increase in inductance due to coupling winding 24. 'I'hus the initial alignment of the device is not disturbed when switch I is thrown vto contact 22, which is the normal operating position. 'I'he width of the selectivity characteristic is largely determined by the degree of overcoupling obtained between circuits 29 and 30, and this in turn is dependent upon the number of turns in coupling winding 24 with relation to inductor 3-4. The steepness of the sides of the selectivity characteristic, however, depends chiefly upon the sharpness of tuning of each of circuits 28, 29 and 30. I

In the modified form of Fig. 3 which is shown in Fig. 4, switch I5 is arranged in such a manner as to cut windings 24 and 25 in series in and out of tuned circuit 29 instead of circuit 30. In this case, coupling winding 24 is wound over coil 5, and bucking winding 25 is coupled to coil 3. The advantage of this arrangement is that the grid or input circuit of the vacuum tube which follows the device is not opened when switch I5 is operated, since the coupling arrangements are switched into and out of the middle tuned circuit. This form is particularly desirable when there is any possibility of damage to the vacuum tube or to circuit components if the tube operates even momentarily without direct-current grid voltage.

The modifications of Figs. 5 and 6 are especially adapted for use ahead of a diode detector or rectifier, as for example the diode portion of a type 75 duplex-diode high-mu triode vacuum tube. In these forms, the coupling is altered between circuits 28 and 29 instead of between circuits 29 and 30. Thus the change in coupling is independent of the loading effect of the diode on tuned circuit 30. The arrangement of Fig. 6 has the advantage over that of Fig. 5, that the plate circuit of the preceding vacuum tube is not opened when switch I5 is operated.

In the arrangement of Fig. 7, the coupling is varied both between circuits 28 and 29 and between circuits 29 and 30. Two coupling windings 24 and two bucking windings 25 are employed, and the two switches I5 are preferably arranged to be operated simultaneously. This modification is advantageously employed where a selectivity characteristic having an extremely wide flat top is desired, as for example in a radio receiver intended only for the high-fidelity reception of strong local stations.

Fig. 8 shows a simplified form of the arrangement of Fig. 7, which is suitable for use in devices to be produced in quantity. In this case, two coupling windings 24 and a single bucking winding 25 are employed, the latter being of sufcient size to oppose the additional inductance which is introduced into circuit 29 due to windings 24 when switch I5 is on contact 22.

Fig. 9 shows a modification of the arrangement of Fig. 4, in which a potentiometer 3I is connected across coupling winding 24 and bucking winding 25, the switch I5 of Fig. 4 being dispensed with. The movable contact 32 of the potentiometer is connected to capacitor I3 to complete tuned circuit 29. The degree of coupling between circuits 29fand 30 may be varied from a minimum value corresponding to critical coupling to a maximum value corresponding to the full degree of overcoupllng provided by windings 24 and 25, by adjustment of the position of contact 32, thus providing continuous adjustment of the degree of overcoupllng and consequently of the fidelity of the receiver in which the device is used.

The use of ferromagnetic cores 2, 4 and 6 increases the sharpness of tuning of circuits 28, 29 and 30 by decreasing the losses of inductors I-2, 3-4 and 5-6 respectively over those realized with air-core coils of similar inductance values, and thus the cores contribute materially to the over-all performance of the device. Although any suitable type of inductor may be employed in the resonant system of the invention, I prefer to employ inductors of the type described in U. S. Patent No. 1,978,568 issued to Crossley and Neighbors. Inductors of this type include magnetic cores made in accordance with U. S. Patent No. 1,982,689 issued to Polydoroff.'

Although the arrangements illustrated in the drawings and described heretofore show adjustable capacitors for tuning of the resonant circuits to a desired frequency, it will be understood that adjustable inductors or both adjustable capacitors and adjustable inductors may equally well be used for this purpose. Thus, in the inductors shown and described the magnetic core may be arranged to be movable relatively to its winding in order to change the inductance of same and thus to adjust the frequency of the circuit.

Further it will be understood that tuned circuit 29 in any of the arrangements of Figs. 3-9 inclusivev may be utilized for additional purposes. Forexample, the voltage across this circuit may be supplied to an additional vacuum tube which operates to regulate the amplification of one or more of the amplifying vacuum tubes of the receiver. Other additional uses for this middle circuit will occur to those skilled in the art.

It will be understood that the function of the coupling winding and the bucking Winding may, in certain instances, be to decrease rather than increase the coupling between the tuned circuits. For example, in an arrangement in which the circuits were originally overcoupled, the coupling Winding and bucking Winding might be inserted in the circuit by the switch for the purpose of decreasing the coupling in order to accurately align the tuned circuits.

It will also be understood that the novel arrangements herein described, while particularly advantageous in coupling systems employing three resonant circuits, are equally applicable to systems employing only two resonant circuits. Thus in Figure 3, the output terminals of vacuum tube 26 might be connected directly to the appropriate terminals of resonant circuit 29, resonant circuit 28 being thus eliminated. Similarly in Figures 4 and 9, resonant circuit 28 would be eliminated by connecting the input directly to resonant circuit 29. In Figu res 5 and 6, output circuit 30 might be eliminated and the amplifying vacuum tube or other load circuit connected directly across resonant circuit 29. In each of the arrangements just described there would result a two-circuit coupling device embodying the invention.

By way of illustrative example of a high-fren quency resonant system arranged in accordance the user from the front panel.

with Figs. 1, 2 and 3' of the drawings, the fol- `lowing data is given for a device intended to be used between a type 6A7 pentagrid converter tube and a type 6D6 triple-grid amplifying vacuum tube at a frequency of 465 kilocycles:

Inductance values (measured at 1000 cycles in a li?" 2" a: 4" aluminum shield) Mutual between circuits 29 and 30 (initial) 11.5 Mutual between circuits 29 and 90 (operating) 21.5 Mutual between circuits 29 and 30 0.25

Performance Position of switch Initial Operating a7 4o 12.5 ke. 16.8 kc. Band width at times 29.2 kc. 36.8 kc.

Although switch I5 is shown in Figs. 1 and 2 as being mounted upon insulating base 9 of the device, it is within the scope of my invention to mount this switch on the front panel of the radio receiver or other apparatus in which the device may be employed, the connections to the switch being extended by means of suitably shielded leads. With such an arrangement, two different degrees of selectivity may be selected manually by the user. Referring to the' modification which is shown in Fig. .'l of the drawings, it is within the scope of the invention to operate switches I5 in sequence instead of simultaneously, in such manner that as many as four different degrees of selectivity are readily obtained, and to so arrange the switches that a choice of the desired degree may be made by Furthermore, it is within the scope of this invention to provide more than two degrees of selectivity by employing means for varying the coupling between two of the inductors in three or more steps. Such means may include additional switch points, with coupling windings and bucking windings for each additional step. Irrespective of the number of selective steps, the device is readily adjusted to resonance at a desired frequency in the manner hereinabove described.

Having thus described my invention, what I claim is:

1. A high-frequency resonant system including plural resonant circuits each having a capacitor and an inductor and adjustable to a desired frequency, two `windings connected in series, and a switch for inserting said windings in series with the inductor in one of said circuits, said windings'being inductively so related to said inductors that by operation of said switch the coupling between two of said circuits may be changed while maintaining the effective inductance values in each of said circuits substantially unchanged.

2. A high-frequency resonant system including plural resonant circuits each having a capacitor and an inductor and adjustable to a desired frequency, and means for altering the selectivity characteristic of said system,v said means including a first winding coupled-to a rst of said inductors, a second winding coupled toa second of said inductors, and a switch for connecting said windings in series with said second inductor in one of said resonant circuits.

3. A high-frequency resonant system including plural resonant circuits each having a capacitor and an inductor and adjustable to a desired frequency, and means for altering the selectivity characteristic of said system, said means including a first winding coupled to a first of said inductors and a second winding coupled to a second of said inductors, said windings being connected in series with said second inductor in one of said resonant circuits, and switching means for eliminating said windings from said resonant circuit, said second winding being so wound and polarized with respect to said second inductor as to counterbalance the effect of said rst winding upon the resonant frequency of said resonant circuit.

4. A high-frequency resonant system including three resonant circuits each having a capacitor and an inductor and adjustable to a desired frequency, and means for altering the selectivity characteristic of said system, said means including a irst winding coupled to a first of said inductors, a second winding coupled to a second of said inductors, said rst and second windings being connected in series with said first inductor in one of said resonant circuits, a third winding coupled to the third of said inductors, a fourth winding coupled to said second inductor, said third vand fourth windings being connected. in series with said second inductor in another of said resonant circuits, and switching means for eliminating said windings from said resonant circuits.

5. A high-frequency resonant system includlng three resonant circuits each having a capacitor and an inductor and adjustable to a desired frequency, and means for altering the selectivity characteristic of said system, said means including a first winding coupled to a first of said inductors, a second winding coupled to a second of said inductors, a third winding coupled to the third of said inductors, said first, second and third windings being connected in series with said second inductor in one of said resonant circuits, and switching means for eliminating said windings from said resonant circuit.

6. A high-frequency resonant system including plural resonant circuits each having a capacitor and an inductor and adjustable to a desired frequency, and means for altering the coupling between two of said circuits while maintaining the effective inductance values in each of said circuits substantially unchanged, said means including a first winding coupled to a rst of said inductors, a second winding coupled to a second of said inductors, and a switch for connecting said windings in series with said second inductor in one of said resonant circuits.

'7. A high-frequency resonant system including plural resonant circuits each having a capacitor and an inductor and adjustable to a desired frequency, and means -for varying the coupling between two of said circuits while maintaining the effective inductance values in each of said circuits substantially unchanged, said means including a first winding coupled to afirst of ,said inductors, a second winding coupled connected in series with said second inductor in one of said resonant circuits, and a potentiometer connected to control the high-frequency current through said windings.

8. A high-frequency resonant system including plural resonant circuits each having a capacitor and an inductor and adjustable to a desired frequency, and means for altering the selectivity characteristic of said system, said means including a rst winding coupled to a rst of said inductors and a second winding coupled to a second of said inductors, said windings being connected in series with said second inductor in one of said resonant circuits, and switching means for eliminating said windings from said resonant circuit.

9. A high-frequency resonant system including plural resonant circuits each having a capacitor and an inductor and adjustable to a desired frequency, and means for altering the selectivity characteristic of said system, said means including a iirst winding coupled to a first of said inductors and a second winding coupled to a second of said inductors, said windings being connected in series with said second inductor in one of said resonant circuits, and a potentiometer connected to control the high-frequency current through said windings.

10. A high-frequency resonant system including plural resonant circuits each having a capacitor and an inductor and adjustable to a desired frequency, and means for altering the coupling between two of said circuits while maintaining the eiective inductance values in each of said circuits substantially unchanged, said means including a rst winding coupled to a rst of said inductors and a second winding coupled to a second of said inductors, said windings being connected in series with said second inductor in one of said resonant circuits, and switching means for eliminating said windings from said resonant circuit.

BERNARD R. scHoFmm. 

